1. Technical Field
The disclosure relates to a pneumatic radial tire.
2. Related Art
In a pneumatic radial tire for the purpose of running rough road, a technique of providing plural projections arranged along a tire circumferential direction on a side wall part is conventionally known (for example, see JP-A-2010-188975). The projections have a function to generate traction by shear resistance when a tire sank in a muddy ground, and are provided for the purpose of securing driving performance on a muddy ground. Furthermore, it is known that external damage resistance performance during running rough road is improved by the projections.
Off-road races include a race of running not only muddy ground and sandy ground, but rocky area such as rocky mountain. In the case of running rocky area, a tire is adjust to have low inner pressure in order to improve road surface follow-up performance by utilizing deformation of a tire. Thus, in the case of running the rocky area with a tire having low inner pressure, particularly in the case of running through rocky mountain composed of piled large rocks having a diameter of 1 meter or more, the tire cannot develop sufficient driving force under grounding conditions that blocks on a tread surface contact the surface of rocks in a form of line contact or point contact. Therefore, to obtain driving performance, grounding conditions are required such that the blocks on the tread surface deform so as to wrap rocks, and contact the surface of rocks in surface contact.
When a tire has low inner pressure, vertical rigidity of the tire is decreased due to the decrease in bending rigidity of a side wall part. However, regarding surface rigidity of a tread, a constant region on a tread surface is relatively gently deformed so as to place a dish bottom upward, due to a hoop effect of a steel belt layer embedded in a tread part. Therefore, for example, in the case of grounding acute-angled rocks, there is a limit to increase wrapping property by the blocks on the tread surface.
It is considered to weaken the hoop effect by reducing the number of lamination of steel belt layers or decreasing a diameter of a steel cord in a belt layer. However, where the hoop effect is weakened, running performance on ordinary paved road and non-paved road, and load resistance are reduced. Thus, there is a limit to weaken the hoop effect. For this reason, it is desired to increase deformation of blocks on a tread surface by adjustment of air pressure, particularly under low inner pressure conditions.
On the other hand, where projections are simply provided on a side wall part as in JP-A-2010-188975, bending rigidity of the side wall part is increased, and the effect that the projections support local deformation of the tread surface is developed. For this reason, wrapping property on the tread surface tends to be deteriorated.
JP-A-2010-264962 discloses a pneumatic tire for running rocky area, wherein a projection in a circumferential direction annularly extending along a tire circumferential direction and plural projections in a radial direction extending from the projection in a circumferential direction to a tire radial direction are provided on an outer surface of a side wall part. However, this document discloses that the projection in a circumferential direction and the projections in a radial direction are provided in order to improve damage resistance of a tire when run-off of the tire has occurred in a rocky area, and does not disclose road surface follow-up performance during running with a low inner pressure tire in a rocky area.
On the other hand, JP-A-61-081207 discloses that vertical deflection is increased by providing plural grooves in a circumferential direction on an outer surface of a side wall part in a wavy shape in cross-section. However, this document has an object to obtain flexing action in the side wall part, and does not aim at local deformation of a tread surface under low inner pressure condition. Actually, in the case of providing the grooves in a wavy shape as in this document, load diffusion is performed to the respective groove parts. Therefore, it is difficult to obtain local deformation on the tread surface. Furthermore, in the case of providing plural grooves in a circumferential direction in the vicinity of a tire maximum width position in a close contact state, deformation may be impaired by that wall surfaces of the grooves in a circumferential direction contact with each other under large deformation.